For many applications, for example, that of photographic film projection, illuminators are required that are able to deliver as much light as possible into a constricted area of specific size and shape. The light being delivered to such an area must have some maximum angular divergence at each point of the illuminated area such that an associated image projection lens can accept the light for projection. To accomplish this, the initial source of light must have high brightness and the optical system associated with the source must collect the light from the source, control its divergence or collimation and provide for its direction to the area to be illuminated. For the best efficiency the divergence should decrease only in proportion to an increase in the area illuminated.
Because of limitations in the brightness of the initial source of light, or limitations in the associated optical system in collecting, collimating and otherwise directing the light to the area requiring illumination, presently available illuminators are often unable to provide the level of illumination needed.
At the present time, the applications requiring the highest source of brightness available are utilizing short-arc lamps. These short-arc lamps have their electrodes rather closely spaced in a relatively large size fused silica envelope. Normally, xenon gas or mercury vapor at high pressure is used as the excitation gas. These short-arc lamps utilize a high current arc of relatively low voltage, i.e., one hundred amperes at thirty volts is typical for a 3000 watt theater projection lamp. Due to the high current utilized in such lamps the anode dissipation is typically one-third of the input power, which causes some inefficiency in the lamp and requires the anode to be of substantial size to permit adequate cooling.
Short-arc lamps of the type discussed above have utilized a variety of different optical systems. In one such system, for example, light sources for theater projection, the lamp is normally surrounded by an ellipsoidal reflector that directs light at the input aperture of the projection lens. A primary requirement is that the film being projected must be fully illuminated by the radiation passing through the film. Since the beam has a circular section, the beam diameter at the plane of the film must be equal to at least the diagonal measurement of each frame of the film passing by the inlet aperture of the projection lens. Because of this it is required to illuminate an area substantially larger than that of the film. Thus, this optical system, although it is relatively one of the best available, is not as efficient nor has as much brightness as one would like. This is due in part to the need to illuminate an area larger than that of the film frames, but, it is also due to aberrations of the particular optical system as well as some limitations in its ability to efficiently collect the radiation from the short-arc lamp.
Another radiation source are tubular arc lamps with a relatively high ratio between outside and inside diameter which results in them being referred to as capillary arc lamps. Such lamps are also relatively long relative to their diameter. They are used in a number of applications where a high brightness source of light is required, however, these lamps do not have as much brightness as the short-arc lamps. Because of this diminished brightness, relative to the short-arc lamps, and because the high aspect ratio of length to width makes it difficult to illuminate a format of moderate aspect ratio, such as film, their applications have been limited.
On the positive side, however, such capillary arc lamps do have the advantages of low arc current and low anode losses; they can be made in a very compact form; and have the advantage of being relatively inexpensive. However, an additional limitation which has restricted the use of capillary arc lamps is their need for rather intensive cooling due to their small size. They are known to be used immersed in flowing water or, alternatively, with a high pressure air blast cooling them. It will be recognized that both of these procedures present problems. In the case of water flow, the water must be kept pure to avoid deposits on the heated lamp. Deionizing resins as well as the use of other carefully selected materials in the water circulating system are required. On the other hand, the use of high pressure air blasts requires pumps of considerable size and generally results in organic vapor decomposition products depositing on the hot lamps after an extended period of operation.
A further major limitation on the brightness of capillary arc lamps is the thickness of their walls which limits the power input that can be tolerated without causing over heating of the inside surface of the tube. (The amount of power input being directly related to the brightness factor as well as the generation of heat by the arc.) The walls must be thick to withstand the stress due to the high internal gas pressure generated at such elevated temperatures. Capillary lamps normally operate with an OD to ID ratio of at least three to one and are generally made of fused silica. The maximum loading normally used is around fifty watts per millimeter of arc length. At this power level the temperature of the inside of the tube wall can be estimated at 1100 degrees to 1200 degrees Centigrade. Fused silica at this temperature has reduced strength and the outer regions of the tube being at lower temperature are under stress induced by the thermal gradient and by the internal pressure. This stress closely approaches the maximum that can be sustained with little margin for error.